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police lightbar power supply

deoxys987

Jan 26, 2012
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i have an old school police lightbar, and the rotators use 17.3 amps constant, and i need to know how much surge amps that 17 amps would have, im looking at getting a 34amp switching power supply. would 34 amps be enough to run it?

i have a 19 amp supply and that can run the low amp functions but i need to know if 34A is enough for surge amps on something that uses 17A

theres 4 rotators that draw 4.3A each.
 
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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4.3A to turn a lamp around? Or does this include the power to drive the lamp?
 

deoxys987

Jan 26, 2012
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4.3A to turn a lamp around? Or does this include the power to drive the lamp?

4.3A to power and drive the lamp. How much would the surge amps be for 4 of these rotators? The amp total would be 17.3 amps
 

(*steve*)

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OK, that's more reasonable. The peak current could be 20 amps or more per lamp.

The (relatively) simple solution is to wire up a power transistor in series with a healthy base current so that it saturates when the lamp is running, but limits the current when the filament is cold.

A fairly simple solution is to have an RC network on the gate of a fairly meaty MOSFET so that it takes perhaps half a second to turn on, effectively limiting the current until the filament is nice and hot.

The power dissipation during turn on would be large, however the average power would be very low. A TO-220 packaged device may have sufficient thermal mass to require only a very small (or perhaps no) heatsink.
 

deoxys987

Jan 26, 2012
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OK, that's more reasonable. The peak current could be 20 amps or more per lamp.

The (relatively) simple solution is to wire up a power transistor in series with a healthy base current so that it saturates when the lamp is running, but limits the current when the filament is cold.

it will shut off the supply. Would a 34 Amp supy work for the rotators? So you're saying a 34A supply would work and power the 4 rotators?
 
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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No it won't shut off the supply.

It will turn it on slowly (like having a huge variable resistor automatically turned from maximum to minimum resistance over some period of time.

Would a 34A power supply be ok?

It depends on how this reduces the peak current demand.

With some trial and error I'm certain it could.

You would determine (by starting with one rotator) what the maximum current required was (the RC time constant would affect this) and if this is less than 1/4 of what your supply can provide, you're good to go.

You would also need another resistor to let the charge leak away. I'll draw something up shortly.
 
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